In a conventional third-generation computed tomography (CT) system a single x-ray source 100 generates a fan beam 102 directed at an extended detector array 104, as shown in the cross-sectional view of FIG. 1. Fan beam 102 has a collection of rays diverging from source 100 at a divergence angle α, as shown. A system of this type, where the fan beam diverges from a single point source to a large array of detectors, is said to have a forward geometry. In an inverse geometry system, the, point source is exchanged for a small array of detectors (or a single detector) and detector array is exchanged for a source array, so that the set of measurement rays converge at the detectors. In the context of the present invention, forward and inverse geometry systems have similar geometrical properties. Thus, the common geometrical properties of both forward and inverse geometries can be described by considering just the forward geometry case.
The rays of the fan beam 102 include a central ray 108 which is defined to be the ray from the point source 100 that intersects a midpoint 110 of the detector array 104. (In the corresponding inverse geometry, the central ray is the ray from the midpoint of the source array to the mid-point of the small detector.) Note that in this conventional system the central ray 108 passes through (or very close to) a rotational axis 106 of the system. During operation of the system, source 100 and detector 104 are rotated around rotational axis 106 to various rotated positions. For example, FIG. 1 shows a rotated position corresponding to a rotation of the central ray 108 by an angle θ. As the source 100 and detector 104 rotate, fan beam 102 also rotates, providing the system with the capability to acquire x-ray transmission data at various angles from which an image is reconstructed. The rotational angles θ must cover a sufficient range so as to allow objects to be properly reconstructed. In this case, the range of θ values must be at least a plus 180 degrees. A field of view (FOV) 114 of the system is the region that is always exposed to the fan beam. Thus, for example, any portion of an object that is positioned within FOV 114 will be viewed from all rotational angles of the system. Outside of FOV 114, however, image data is not available at some rotational angles. As a result, CT systems are designed to reconstruct three-dimensional representations of objects within the FOV of the system. (Here the FOV is the in-plane FOV, i.e., the FOV within the cross-sectional plane of the fan beam which is perpendicular to the rotational axis.)
In the conventional CT system shown in FIG. 1 the size of FOV 114 is limited by the size of the detector array 104. In particular, the diameter of FOV 114 is always significantly less than the extent of the detector array. An increased FOV can be provided by increasing the size of the detector array, as shown in FIG. 2. A source 200 emits a fan beam 202 toward a larger detector array 204. Fan beam 202 has a central ray 208 which passes through (or very close to) rotational axis 206 and intersects a midpoint 210 of detector array 204. Due to the increased size of the detector array 204, the system has an increased FOV 214 as compared to the smaller FOV 212 provided by the smaller detector. (Similarly, an inverse geometry system also has an increased FOV if it has an increased source array size.) Although the FOV of a CT system can be increased using a larger detector array, increasing the size of the array often introduces significant technical difficulty and expense.
Another drawback of this CT system design is that the source and detector must rotate through a large angle to acquire images from a sufficiently large range of angles. If a patient moves during the rotation, the image data from different angles will not be consistent, resulting in artifacts and errors in the reconstructed three-dimensional representation. Alternative CT system designs (such as U.S. Pat. No. 5,966,422 to Dafni et al. and U.S. Pat. No. 4,196,352 to Berninger et al., which are incorporated herein by reference) have been proposed in an attempt to overcome this disadvantage. For example, FIG. 3 shows a CT system with multiple sources 300, 302, 304 and multiple corresponding detector arrays 306, 308, 310. The sources 300, 302, 304 emit corresponding fan beams 312, 314, 316 having respective central rays 318, 320, 322 all intersecting at a point coincident with (or very close to) an axis of rotation 324. Because the three source and detector pairs simultaneously provide image data at different angles, the required rotational angle is reduced by three, helping to mitigate problems caused by patient movement during scanning. However, the field of view 326 of this system suffers from the same problem as the conventional single source-detector system of FIG. 1. To increase the FOV of this system, the detector array sizes must be increased. In any case, the FOV is always less than the detector size. Moreover, despite the use of three detector arrays and sources, there is no FOV increase compared to the single source-detector system of FIG. 1. (The same disadvantages apply to the analogous inverse geometry system.)
An alternative CT system that provides a slight increase in FOV is shown in FIG. 4 (see also U.S. Pat. No. 5,430,297 to Hawman, which is incorporated herein by reference). A single source 400 emits a fan beam 402 directed at a detector array 404. A central ray 406 of fan beam 402 intersects a midpoint 410 of detector 404. In contrast to the conventional system of FIG. 1, however, the fan beam 402 of source 400 is offset from centerline 412 so that the central ray 406 is offset from the rotational axis 408 of the system. Consequently, line 418 from source 400 passing through axis 408 intersects the detector 404 at a point 420 that is far from midpoint 410. As the source 400 and detector 404 rotate around rotational axis 408, the single fan beam also rotates around axis 408. Due to the offset of the fan beam, the FOV 414 of this system is larger than the FOV 416 of a comparable system with no offset, provided the system rotates through at least 360 degrees. The FOV 414, however, while larger than FOV 416, is still substantially limited unless the detector array is quite large. In particular, the diameter of the FOV of this system is always less than twice that of the system of FIG. 1, and generally less than the extent of the detector array. Moreover, the asymmetry of the system geometry requires a rotation of at least 360 degrees, introduces complexities to the data processing required to reconstruct a representation of the object from the data collected at various angles, and in general has non-uniform noise behavior. (The analogous inverted system has similar limitations.)